Adjustable bipolar voltage regulator

Encyclopedia of radio electronics and electrical engineering / Surge Protectors

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If you have a bipolar power supply, supplement it with the suggested voltage regulator. And then you will become the owner of a source whose output voltage can be adjusted over a wide range, and not worry about the appearance of short circuits in the load - the automatic protection will work instantly.

The stabilizer (see figure) is intended for use in a laboratory power supply and provides a bipolar stabilized voltage that can be adjusted with one resistor in the range from 1,5 to 20 V at a load current of up to 3 ... 5 A. The stabilizer is equipped with load current protection , the value of which can be smoothly adjusted from maximum to 70 mA. A critical situation is signaled by a light indicator.

The stabilizer is assembled on the basis of the K142EN6A microcircuit - a bipolar voltage stabilizer. To expand the range of output voltage adjustment, its inclusion is slightly changed compared to that recommended in the article by S. Biryukov "Option for switching on the K142EN6 microcircuit" ("Radio", 1996, No. 12, p. 47).

As you know, the output current of this microcircuit is limited to 200 mA, which is clearly not enough for a laboratory power supply. Therefore, to increase this parameter, powerful transistors are introduced into the stabilizer: VT2 - into the positive power bus, VT6 - into the negative one. On transistors VT3, VT7, the current limiters of the microcircuit are assembled, and on the LEDs HL1, HL3 - emergency mode indicators.

Transistors VT1, VT5 act as short circuit current limiters, which can be set by variable resistors R2 and R6 in each bus independently of each other. Transistors VT4, VT8 work as current stabilizers for LEDs HL2 and HL4, respectively. This is necessary so that the brightness of the LEDs is maintained constant when the output voltage changes over a wide range. Regulate the output voltage on both outputs with one resistor R10.

The stabilizer works like this. At low output currents (15 mA or less), transistors VT1, VT2, VT5, VT6 are practically closed and the entire output current flows through the microcircuit, transistors VT3, VT7 and resistors R1-R3, R5-R7. The voltage drop across VT3 and VT7 is small, and the LEDs HL1, HL3 are off. With an increase in the output current above the specified, transistors VT2 or VT6 begin to open and the output current flows mainly through them, bypassing the microcircuit.

The stabilizer will operate in this mode until the load current exceeds the value set by resistors R2, R6. If this happens, then the transistor VT1 (VT5) will open and the current that flowed through the emitter junction of the transistor VT2 (VT6) will flow through the collector circuit of the transistor VT1 (VT5). The output current will be limited. In this case, the current through the transistor VT3 (VT7) will increase, the voltage across it will increase and the LED HL1 (HL3) will light up, signaling an emergency mode.

In the event of a short circuit at the output, the processes will be similar, but at the output where the accident occurred, the LED will turn off.

To power the stabilizer, a transformer and rectifiers of the appropriate power are required. In addition, since the stabilizer is adjustable, it is desirable to use a transformer with a switchable output voltage, which will facilitate the thermal regime of transistors VT2 and VT6.

The device can use transistors KT814A-KT814G, KT816A-KT816G (VT1), KT825A, KT825G-KT825E (VT2), KP302V, KP302VM (VT3, VT7), KP303V-KP303D (VT4, VT7), KT815A-KT815G, KT817A-KT 817G (VT5), KT827A-KT827V, KT829A-KT829G. LEDs - any of the AL307, AL341 series, preferably of different colors for different output voltage polarities. Capacitors - KM, K73; resistors R1, R5 - constant wire; R2, R6 - variable wire with a power of at least 1 W (if it is difficult to purchase such, it is permissible to replace them with a set of fixed wire resistors with a switch for several positions); R10 - SP, SPO, SP4.

Establishing a stabilizer comes down to setting the maximum allowable short-circuit current with resistor R1 (R5). The current of the microcircuit, at which the transistor VT2 (VT6) starts to open, can be selected by the resistor R3 (R7).

To increase reliability, it is useful to include resistors with a resistance of 1 ... 5 Ohms between the base terminals of transistors VT2 and VT3 and the connection points of resistors R6, R7 and R150, R220, respectively.

Author: I.Aleksandrov, Kursk

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